This invention relates to focus detecting devices, and more particularly to a focus detecting device applied to a camera.
A variety of methods are known in the art, in which focus detecting is carried out by utilizing variations of object contrast, spatial frequency and light quantity according to focal conditions. In a first conventional method, a photo-electrical element made of CdS having a particular contrast responsive characteristic is employed. In the second method, a photo-electrical output signal which is obtained by mechanically vibrating or rotating an optical member. In the third method, similar to a range/view finder, two outputs of light detecting members provided at different positions are used to detect the completion of focus adjustment.
The first method has several disadvantages in that it is difficult to manufacture photo-electrical elements having the same light responsive characteristic. It is also difficult to detect the focus adjustment completion in the case of low light levels because variations in output of the photo-electrical element are small before and after the focal point is detected. The first method is also disadvantageous in that it is difficult to miniaturize a camera using this system because it is necessary to provide mechanically moving components. Also a relatively large amount of current is consumed if the moving components are driven electrically.
The third method has a drawback that it is difficult to exchange the lens. Contemporary cameras use interchangeable lens making this technique not commercially feasible in a large number of contemporary designs.